Noise components mainly caused by image sensors are superposed on digital images obtained with a digital camera and so on. This noise component is one of the reasons for deteriorating image quality. Generally, efforts are being made to improve the image quality by reducing the noise components superposed on the image using various methods.
Granular noise caused mainly by silver halide particles arises also in silver halide photographs and is one reason for the deterioration of image quality as in digital images. However, the granular feeling caused by the granular noise in fact has some value as an expression of silver halide photographs.
Under such a situation, in order to give the same value of the silver halide photograph to digital images and to enable the adding of the visual effects of silver halide particles, various types of technologies for superposing the granular pattern imitated to the granular feeling of the silver halide particles onto digital images are being proposed. For example, in U.S. Pat. No. 5,641,596 B1, a random number is generated per pixel, and frequency filter processing and scale changing are performed to superpose the granular pattern having expected granularity onto the image. Moreover, in JP11-85955A, a granular pattern is obtained by subtracting a smoothed image from an exposed image obtained from color film exposed uniformly.